Electrolytic methods of producing various kinds of useful materials by electrolyzing water or by electrolyzing an electrolytic solution obtained by dissolving an electrolyte in water hitherto have been widely practiced. The development of these electrolytic methods has largely changed the production steps of some conventional products.
For example, for the washing step in the production of semiconductor devices or liquid crystal panels, hitherto, an organic solvent such as trichloroethane, tetrachloromethane, etc., an inorganic solvent such as hydrofluoric acid, sulfuric acid, hydrochloric acid, nitric acid, etc., or an oxidizing agent such as ozone water, hydrogen peroxide water, etc., was frequently used. However, these chemicals are not only dangerous to use, but there also are problems in that an organic solvent can cause environmental pollution such as disruption of the ozone layer, etc. In the case of an inorganic acid or a salt, many problems and costs are associated with the treatment of the waste water. Furthermore, the semiconductor devices or the liquid crystal panels which are subjected to a washing treatment using these chemicals require a large amount of super-pure water for removing chemical residues therefrom.
Moreover, besides the above mentioned semiconductor devices and liquid crystal panels, in medical treatment, food industries, etc., there are problems in that a large amount of detergent is used for sterilizing and washing. Thereafter, the detergent must be washed off with large amounts of water.
For solving these problems, a method has recently been proposed which comprises electrolyzing water or an aqueous solution containing a small amount of hydrochloric acid and a salt such as sodium chloride, ammonium chloride, etc., in an electrolytic cell partitioned with a diaphragm into an anode chamber and a cathode chamber. An aqueous solution having a high oxidation reduction potential (ORP), that is, a very high oxidative property and also a slightly acidic property is formed in the cathode chamber. On the other hard, an aqueous solution having a low ORP, that is, a very strong reductive property and also a slightly basic property is formed in the cathode chamber. These solutions are used for washing semiconductor devices, liquid crystal panels, etc., as described above.
When electrolysis is carried out by adding a small amount of sodium chloride to an electrolytic solution, acidic water having a strong sterilizing action is obtained. The sterilizing action of the acidic water is utilized in food production and for medical treatment. However, the ORP of the acidic water is high, which indicates that the chloride ion in the liquid is converted to hypochlorous acid. The hypochlorous acid can induce the formation of an organic chloride. Thus, there is a possibility of causing secondary pollution, although the possibility is very low. When the foregoing acidic water is used for washing semiconductor devices or liquid crystal panels, there is a possibility of secondary pollution as described above.
Titanium electrodes covered with platinum are usually used in this type of electrolysis. The electrodes are consumed at a rate of from about 1 to 10 .mu.g/AH, and when these electrodes are used in an electrolytic solution for electrolysis, from 1 to 10 ppb of platinum is dissolved and mixed in the electrolytic solution. Also, when an aqueous solution of about 100 ppm of hypochlorous acid is prepared by electrolysis, an oxide electrode such as iridium oxide, etc. is used, and the electrode is consumed at a rate of about 1/10 that of platinum.
The amount of the dissolved metal causes no problem in food production and in medical treatment, but is detrimental for washing semiconductors such that removing the metal becomes a large problem.
The inventors have previously succeeded in reducing the consumption of an electrode substance by a factor of about 1/10 by carrying out electrolysis using an ion exchange membrane as a solid electrolyte and closely attaching an electrode to the membrane. However, even in this case, dissolution of a metal which becomes electrically conductive when dissolved in a liquid is a problem although the dissolution amount thereof is slight.
The use of ozone water has been proposed for avoiding this problem. Ozone is widely used as a strong oxidizing agent in various fields such as water treatment for washing semiconductor devices and liquid crystal panels, medical treatment, and in the food industry as described above. Ozone is mainly produced by an electrolytic method which can produce ozone at a high concentration. Also, by the development of quality electrode materials, electrolytic conditions, etc., ozone is produced with good efficiency as described, e.g., in S. Stuck et al., Journal of Electrochemical Society, Vol. 132, No. 2, p. 3382 et seq. (1985), U.S. Pat. No. 4,541,989, and JP-B2-44908 (the term "JP-B" as used herein means an "examined published Japanese patent application"). However, even in electrolytic ozone production, there are problems in that when a metal electrode is used, the metal is dissolved. Furthermore, when a carbon electrode is used, the electrode consumption is severe, which is unsuitable for operation over a long period of time.
To avoid contamination by the dissolved metal from a metal electrode, a non-metal type electrode may be used. Carbon is one such material that is capable of being used as a non-metal electrode.
However, because a carbon electrode is usually porous, the destruction and dissolution of the electrode tend to occur with the progress of the electrolysis. Also, when a carbon electrode is used as an anode, part of the electrode is oxidized to form a carbonic acid gas such that consumption of the electrode is accelerated. Also, even when a carbon electrode is used as a cathode, there is a problem in that the size of the hydrogen bubbles thus formed is smaller than that of oxygen at the anode side. Consequently, the electrode tends to be destroyed even though the electrode is not consumed as a carbonic acid gas. To prevent destruction of the electrode, a large electric current cannot be passed therethrough which inevitably causes a problem in that a large ORP is not obtained.
Besides the electrode for producing acidic water and alkaline water by electrolysis and further the electrode for producing ozone by an electrolysis as described above, there are metal electrodes which can be used in a corrosive atmosphere such as sodium chloride electrolysis, etc. In particular, a dimentionally stable electrode or anode (hereinafter abbreviated as DSE or DSA) is prepared by applying an electrode substance containing a platinum-group metal oxide such as ruthenium oxide onto the surface of a valve metal mainly composed of titanium. The DSE was first practically used for sodium chloride electrolysis and at present, almost all electrodes for sodium chloride electrolysis have been replaced with DSE electrodes throughout the world.
Also, DSE electrodes are useful in the field of high-speed plating, etc., which is accompanied by the generation of oxygen. Because the DSE is stable and does not deform, this electrode can be used at a reduced distance between electrodes. Also, because its overvoltage is low, and furthermore because it can hardly cause environmental pollution, the DSE electrode has been widely used in place of a conventional lead electrode. Besides these uses, the foregoing DSE has been used for waste water treatment by removing COD, the synthesis of organic or inorganic compounds by electrolytic oxidation, etc.
The foregoing DSE electrode can attain a remarkable improvement in electrolytic efficiency. However, the same characteristics which improve electrolytic efficiency can cause other problems. That is, a DSE electrode comprises a corrosion resisting valve metal base material. The valve metal base material does not corrode in many electrolytic solutions and can stably function. However, it sometimes occurs that a part of the base material is not sufficiently stable. That is, the foregoing DSE is usually produced by a thermal decomposition method. It sometimes occurs that the surface of the above-described base material is insufficiently covered with a substance, which is decomposed and attaches to the surface of the base material. An electrolytic solution which contacts the base material through the electrode substance causes a reaction, such that dissolution of the base material is not sufficiently restrained.
For example, when the forgoing DSE is disposed as an anode in an electrolytic bath filled with an organic compound such as methanol, ethanol, etc., and an electric film is applied, titanium, which is the base material metal, is corroded and the electrode substance is released. Also, when the electrolytic bath contains a halogen such as fluorine, bromine, etc., anodic polarization occurs which causes a so-called pitting corrosion or activated dissolution, such that the electrode life becomes very short.
As a counterplan, niobium or tantalum valve metals having a higher corrosion resistance than titanium are used as the base material metal. However, there metals are very expensive and hard to work, the surface thereof is very easily oxidized, and further a surface oxide tends to release from the metal. Thus, in DSE electrodes produced by forming an electrode substance on the surface of the base material metal by thermal decomposition, the treatment condition is largely restricted and at present the use range thereof is very limited.
DSE electrodes are excellent in view of energy savings, the overvoltage for generating chlorine is almost zero, and the overvoltage for generating oxygen is 500 mV or lower. Thus, when using a DSE, chlorine and oxygen tend to evolve, but the reactivity for electrolytic oxidation to a specific material and a decomposition reaction by electrolysis is weak due to the low electrolytic voltage. Therefore, a DSE is scarcely used for practical anodic oxidation.
As a counterplan, a platinum-plated electrode is partially used, but there are problems in that the electrode is very expensive and the life thereof is not always sufficient.
As another electrode, a lead oxide electrode which is scarcely consumed depending on the electrolysis conditions and having excellent oxidative power is also used. However, the lead oxide electrode is disadvantageous in that the electrode must always be anodically polarized in an electrolytic solution. This gives rise to a maintenance problem. Also, in an aqueous solution containing a halide ion, the electrode does not always show good durability.
Furthermore, as a high-overvoltage electrode particularly for electrolyzing an organic compound, a tin oxide electrode has been proposed. Because this electrode is reported to have a very high oxygen-generating overvoltage, the decomposition of an organic compound by anodic oxidation in an aqueous solution is possible, and the electrode is particularly suitable for decomposing a benzene nucleus. However, there are problems in that the electric conductivity of tin oxide itself is relatively low, such that a large current density cannot be obtained. Also, because the electrode is produced by a sintering method, a metal which becomes the core material is hard to set.
Recently, a diamond imparted with electric conductivity has been developed. Since the diamond has excellent heat conductivity, optical transmittance, and durability to high temperatures and oxidation, and in particular because the electrical conductivity can be controlled by doping, the diamond is promising as a semiconductor device and as an energy-conversion element. However, use of the diamond as an electrode for electrolysis has been scarcely reported. Swain et al. reported the stability of diamond in an acidic electrolytic solution and suggested that diamond was far excellent as compared with other carbon materials as described in Journal of Electrochemical Society, Vol. 141, 3382.sup..about. (1994). Also, Fujishima et al., considered the application of diamond as an electrode for a reduction reaction in view of its wide band gap of 5.5 eV as described in Journal of Electroanalytical Chemistry, Vol. 396, 233.sup..about. (1995) and Denkikagaku (Electrochemistry), Vol. 60, No. 7, 569.sup..about. (1992). Furthermore, a humidity sensor utilizing the change in surface resistance of diamond with a change in humidity was also reported in Denkiron (Electric Theory), Vol. 114, No. 5, 413.sup..about. (1994).
However, the industrial utilization of diamond in a high-potential region capable of generating oxygen and capable of generating chlorine in the case of high current density has not yet been reported.